This invention relates to load pull testing of power transistors using automatic microwave impedance tuners in order to synthesize reflection factors (or impedances) at the input and output of said transistors (DUT) and test said DUT under controlled impedance conditions. In addition this invention addresses the matter of supplying DC Bias-to said transistors.
A popular method for testing and characterizing microwave components/transistors for high power operation is “load pull” and “source pull”. Load pull or source pull are measurement techniques employing microwave tuners and other microwave test equipment. The microwave tuners in particular are used in order to manipulate the microwave impedance conditions under which the Device Under Test, DUT, or transistor, is tested (FIG. 1). A load pull setup comprises essentially a signal source (1), a directional coupler (2), one or two impedance tuners (3, 4), two power meters (5, 6) and a DC power supply (7) with associated bias networks (8, 9). It also comprises a test fixture to mount the device under test (DUT, 10).
In a typical setup, as shown in FIG. 1, the bias networks are connected in series with the tuner's idle ports (28, FIG. 4). This is the port towards the signal source (1) at the input tuner and the port towards the load (5) at the output tuner. The main reason for placing said bias networks (8, 9) at those places in the setup is the insertion loss of said bias networks. A bias network, also called a “Bias-T”; FIG. 3, comprises a series DC blocking capacitor (21) and a parallel choc inductor (18). It separates the DC bias supply to the DUT from the RF signal. The RF+DC port (19) of the Bias-T is connected to the DUT whereas the RF port (20) is connected either to the signal source or the RF load. The DC port (17) is connected to the DC power supply.
The Bias-T creates insertion loss in the signal path. This is due to the dielectric loss of the series capacitor (21), as well due to signal leakage into the inductor (17). In addition to those two inherent loss sources, the two adapters at ports (19), (20) and FIG. 2, cause RF signal loss. All together, typical, commercially available Bias-T, (FIG. 2) introduces insertion loss between 0.5 and 2 dB. If the Bias-T's were inserted between DUT and tuners (FIG. 1) instead between tuner and source/load, their insertion loss would reduce the tuning range of the tuners (FIG. 5). The tuning range (35) is the area of the Smith chart (33) that can be covered by the tuners at the test port facing the DUT (32); if no bias network is inserted between tuner and DUT, a typical tuning range is shown as (35) in FIG. 5. If a Bias-Tee were inserted between tuner and DUT then the tuning range would be reduced to the area described by the circle (34). In general the main reason for the insertion loss of Bias-T's are the two coaxial mainline connectors needed (FIG. 2). The internal parts of said bias networks (18, 21) generate only part of said insertion loss. One of the benefits of this invention is reducing insertion loss by incorporating said bias networks inside the tuners and avoiding said mainline connectors, which allows inserting Bias-T's between tuners and DUT for better intermodulation performance and spurious oscillations [2], [5].